In many large electronic equipment centers or data centers it is common practice to mount electronic equipment in enclosures called “racks.” In order to cool the equipment, the racks have open fronts and backs and are placed in an air-conditioned computer room. The interior of a typical computer room 100 is illustrated in FIG. 1. In such a room, a raised floor 103 is constructed over the actual floor 102. The raised floor is conventionally constructed of a grid framework 104 that supports modular floor panels 106. The racks of equipment, of which racks 116, 118, 120, 122 and 124 are shown, are placed on the raised floor 103. Typically, channels 130 and 132 run across the tops of the racks to allow the electronic equipment to be interconnected by cables. (not shown) that are placed in the channels.
At the perimeter of the room 100, one or more computer room air-conditioning (CRAC) units, of which unit 108 is shown, provide cooling. Unit 108 generates a stream of refrigerated air illustrated schematically by arrow 110 in the space between the raised floor 103 and the actual floor 102. The cooled air enters the room though perforations 112 in the panels located between the racks 116 and 124 and 120 and 122. The electronic equipment in the racks typically has air movers comprised of blowers or fans that circulate air over the equipment. Thus, the cooled air is drawn into the equipment as schematically illustrated by arrow 114.
The heated air is exhausted from the back of the racks and rises to the ceiling of the room 100 as schematically illustrated by arrows 140, 142 and 144. Warm  air at the ceiling is then drawn into the intake 160 of the CRAC unit 108 as indicated schematically by arrows 150, 152 and 154 in order to begin the cycle again.
While this arrangement operates in a satisfactory manner, computer room personnel must take care to physically locate the equipment racks in the room in order to evenly spread the cooling load represented by the electronic equipment. Otherwise localized “hot spots” can develop and cause equipment overheating even though the CRAC units can handle the overall cooling load. In a large data center where equipment is constantly being added, removed and changed, the logistics of maintaining an even cooling load can become difficult. Further, conventional computer room cooling systems have been designed to handle power dissipations of 1–1.5 kilowatts per rack. However, the reduction in electronic component size has resulted in electronic systems currently being built that dissipate 12–20 kilowatts per rack. Even with proper equipment placement, such systems can cause a hot spot and a cooling overload. If an electronic system overheats, it can become damaged. Alternatively, on-board sensors may power down the system, resulting in an unscheduled service interruption.
One prior art attempt to solve the hot spot problem is to mount a dedicated cooling system on the top of a rack. This dedicated cooling system can be a self-contained air conditioning unit or it can be a heat exchanger that receives a cooling liquid, such as chilled water, via pipes from a cooling system located at the perimeter of the room. The dedicated cooling system draws in hot air rising from the back of the rack, cools the air and generates a stream of cooled air that is discharged in front of the rack where it can be drawn into the rack by the air movers that are part of the electronic equipment. A problem with this system is that there is little control over the cooled air since the racks are open.1
Another alternative is to enclose the electronic equipment in a sealed enclosure that is provided with its own dedicated cooling system. This cooling system can also be a self-contained air conditioning unit or a heat exchanger that receives a cooling liquid from a cooling system located at the perimeter of the room. A sealed enclosure has the advantage that it is completely independent from any of the other  racks in its vicinity. Consequently, such sealed units can be placed anywhere in the computer room without developing hot spot problems. This greatly simplifies the logistics of maintaining the computer room. However, if the cooling system fails, the electronic equipment in the sealed enclosure will rapidly overheat and shut down. One prior art method of solving this latter problem is to include a backup cooling system in the enclosure that can prevent the equipment from overheating at least for a time period long enough to allow the cooling system to be replaced or repaired. Unfortunately, backup cooling systems add significantly to the cost of the enclosure, rendering their use prohibitive in many applications.